The invention relates to an apparatus for the production of monocrystals by a potless zone melting process in which the monocrystal has a shape which, starting out from a seed crystal, passes through a constricted portion or "neck" and a transitional zone and on into the essentially cylindrical main body of the crystal, the said apparatus consisting of a chamber for the formation of a vacuum or of a shielding gas atmosphere, a heating apparatus for the production of the melting heat, driving means for the mechanical movements necessary for the formation of the crystal, a mounting for the seed crystal, and a supporting device which can be applied to the monocrystal at a point beyond the neck and consists of a plurality of gripping bodies adapted for radial engagement with the substantially cylindrical surface of the monocrystal and distributed about its circumference.
In the production of monocrystals by the potless zone melting process, the extremities of the rod from which the monocrystal is formed are generally rotated. This is accomplished by a corresponding revolving of the crystal and rod clamping means. In case a change is produced in the cross section, the clamping means must also be able to perform axial movements.
To start the production of the monocrystal, a crystal known as a seed crystal is used, on which the monocrystal will be grown. Inasmuch as a crystal growth free of dislocations is required, a thin neck must be drawn out at the beginning of the pulling process, after growth has been started on the seed crystal. In this manner, dislocations in the crystal lattice are able to "grow themselves out." The thin neck, which is adjoined by the transitional zone, also called the shoulder, has, in practice, a diameter between about 2 and 4 mm; it is thus the weakest link between the clamp and the monocrystal in formation.
The diameters of the finished monocrystals are usually greater than 50 mm, and sometimes even greater than 80 mm. The length achievable today amounts to about 800 to 1000 mm. For reasons of economy, the largest possible crystals are desired, i.e., crystals of greater diameter and greater length. The attainment of this objective is considerably hampered by the apparatus known today for the following reasons.
The rotation of the crystal produces vibrations due to virtually unavoidable out-of-round growth of the crystal. This creates the danger that, if the vibrations are too great, the crystal structure will be influenced in such a manner that the required freedom from dislocations will be lost. Above all, however, the danger exists that the vibrating crystal will come in contact with the heating device or will break off at the neck, thus immediately stopping the process. The desirable increase in the length and the diameter of the crystal is therefore greatly limited by the low strength of the neck. In addition, a higher speed of rotation of the crystal is also desired, so as to improve the distribution of the doping substances in the crystal lattice. The increase of this speed, too, is limited by the thin neck, since any imbalance would result in the early breaking of the neck.
German "Offenlegungsschrift" No. 2,348,883 has disclosed an apparatus similar to the kind initially described, in which the supporting means consists of at least two jaws movable axially, i.e., parallelly to the crystal axis, which in their operation engage the conical part of the monocrystal being formed. In practice, however, this conical portion has by no means an ideal conical shape, but instead has a considerable amount of surface irregularities which are mentioned in the specification. To compensate for these irregularities, the jaws are covered with an overlay of a compressible material such as graphite felt. Such a material, however, is by no means capable of compensating all of the surface irregularities encountered in practice to such an extent that no transverse forces will be exercised on the conical portion and thus on the neck. It is therefore admitted in the specification that a slight transverse movement of the conical portion may be produced by the supporting means. Experience has shown that, due to the brittleness of the material being used, even a slight transverse movement often produces breaking of the neck. The prior-art apparatus has the additional disadvantage that the points of engagement of the supporting means are located virtually at the lowermost point on the monocrystal, so that the result is a supporting action that diminishes with the increasing length of the monocrystal. This is explained by the continuously deteriorating leverage exercised by the jaws on the monocrystal.
The most important disadvantage of the prior-art apparatus, however, is to be seen in the fact that the supporting means transmits a considerable axial component of force to the monocrystal, the reaction force of which is applied precisely to the thinnest cross section of the monocrystal, namely the neck. It is therefore stated quite correctly in the above-mentioned specification that the pressure exercised by the supporting means must not exceed the maximum allowable tensile stress in the smallest cross section between the seed and the conical portion. This maximum permissible tensile stress, however, is by no means a precisely defined magnitude, and furthermore it is foreseeably diminished by the bending moments exercised on the neck due to the surface irregularities in the conical portion. The danger of the breaking of the neck and hence of the interruption of the growth of the crystal is thus unacceptably increased.
On the basis of German Pat. No. 1,210,415, U.S. Pat. No. 2,876,147 and German Auslegeschrift No. 1,282,601, it is known, in the zone melting process, to guide the formed monocrystal between radially urged transport rolls, i.e., to set it in rotation and at the same time to pull it continuously in the axial direction. The rolls in this case have an entirely different purpose, namely to center the rod as accurately as possible during its continuous movement. Thus, for example, the upper, solid part of the monocrystal is to be held in correct alignment by means of the guiding rolls, which are resiliently urged against one another (German Auslegeschrift No. 1,282,601, col. 3, lines 52-58). The previously known apparatus with positive centering of the monocrystal do not serve for the zone melting of monocrystals which are joined to the seed crystal by the above-described thin neck. Consequently, the problem of breakage at a point on the neck is not involved at all. The use of the previously known apparatus in the production of monocrystals in accordance with the generic concept of the present invention would very quickly result in the breaking of the neck.
On the basis of U.S. Pat. No. 3,002,821, an apparatus is known for the continuous drawing of a monocrystal from a melting crucible, in which apparatus the monocrystal is pulled by two sets of radial gripping jaws in a kind of pilgrim step movement. These jaws, too, produce a positive centering of the monocrystal, which is possible only because the monocrystal has not been reduced in cross section by a neck.